Position detecting systems using electromagnetic coupling methods have been developed as input devices or the like for CAD systems, computers, mobile phones, and other similar applications. In this type of system, a position indicator and a position detecting apparatus transmit/receive signals to/from each other by electromagnetic coupling, so that a position indicated by the position indicator is detected by the position detecting apparatus.
For example, as described in Japanese Unexamined Patent Application Publication No. 5-88811, prior art systems include a design in which a transmission coil is provided at the peripheral portion of a sensor area of a position detecting apparatus, so that one transmission coil is provided at the peripheral portion.
FIG. 8 of Japanese Unexamined Patent Application Publication No. 5-88811 schematically shows the configuration of a sensor unit of the position detecting apparatus. In FIG. 8, a transmission coil 801 is provided around a plurality of sensor coils 802. In order to simplify the figure, only sensor coils 802 in the X direction (X-direction sensor coils) are shown, and sensor coils in the Y direction (Y-direction sensor coils) are not shown.
In FIG. 8, position-detecting signals are transmitted from the transmission coil 801 to a position indicator (not shown) by electromagnetic coupling, and then the plurality of sensor coils 802 are sequentially selected and driven. Accordingly, the plurality of sensor coils 802 sequentially receive position indicating signals from the position indicator through the electromagnetic coupling, so that the position of the position indicator is detected based on detection signals received by the plurality of sensor coils 802.
In the position detecting system described in Japanese Unexamined Patent Application Publication No. 5-88811, a transmission circuit for position detecting signals used for detecting a position can be advantageously simplified by using the single transmission coil 801.
The above-described known position detecting system has an advantage of utilizing a simple transmission circuit for driving the transmission coil 801. However, when the position indicator is positioned near a plane provided with the transmission coil 801 and the sensor coils 802 (sensor plane) but far from the transmission coil 801, the position indicator is difficult to excite. Further, a shield member is provided under the transmission coil 801 and the sensor coils 802 in order to prevent a disturbance of the magnetic field associated with the sensor coils 801, 802. Therefore, a magnetic flux output from the transmission coil 801 flows into the shield member, and thus the position indicator is difficult to excite.
Also, as in a case where the position detecting system is mounted on a mobile phone or the like, when the transmission coil 801 and the sensor coils 802 are laminated on a display device such as a liquid crystal display device, the amount of drive noise of the display device is large at the end portion of the area (sensor area) provided with the transmission coil 801 and the sensor coils 802. Therefore, excitation efficiency of the position indicator should be increased in order to eliminate the effect of the noise. In particular, an exciting current having an opposite phase to that of a current in the transmission coil 801 (exciting current flowing in the opposite direction to that of the current in the transmission coil 801) flows through a metallic bezel, which is provided at the periphery of the display device for attaching the display device. Accordingly, a magnetic flux in the opposite direction to that of the excitation for the position indicator is generated, which causes an undesirable decrease in the excitation efficiency.